I knew that config should work, but no joy.
With a whole lot of trial & error elimination, I finally narrowed it down as far as the user profile in Firefox (a couple of them simultaneously). Given that, I figure it must have been an extension common to each of the user profiles, that broke something when upgrading the last time.
Turning-off all the extensions didn't work either, so I just recreated the user profile from scratch. A pain in the rear, but it worked.